Cerium oxide, which has oxidation-reduction performance, is used for improving the catalytic activity of three-way catalysts comprising at least one platinum group element such as platinum, rhodium or palladium and capable of simultaneously eliminating hydrocarbons(HC), carbon monoxide(CO) and nitrogen oxides(NOx) in the exhaust gas discharged from internal-combustion engines of automobiles, among others.
It is known, however, that catalysts comprising a platinum element and cerium oxide undergo marked reduction in oxidation-reduction performance of cerium oxide and the performance as catalysts readily deteriorate at high temperatures above 800.degree. C.
For maintaining the oxidation-reduction performance of cerium oxide at high temperatures, methods have been proposed which comprise adding an oxide of a rare earth metal other than cerium and/or zirconium oxide to thereby suppress the crystallization of cerium oxide (e.g. Japanese Unexamined patent Publication Nos.58347/1989, 116741/1988). Methods comprising preparing a mixed oxide comprising cerium oxide and zirconium oxide in advance and using the same (e.g. Japanese Unexamined Patent Publication Nos.168544/1987, 281144/1989, 284875/1992).
Further, for increasing the function as a promoter or support(carrier) of a catalyst for exhaust gas purification, methods of improving the thermal stability of specific surface area and the oxidation-reduction performance of a mixed oxide comprising cerium oxide and zirconium oxide have also been disclosed (e.g. Japanese Unexamined Patent Publication Nos.55315/1992, 286772/1993, 278444/1997, and Japanese Patent No.2698302). Today, when advanced measures are being taken to protect the earth environment, the durability temperature with respect to the thermal stability of specific surface area and the deterioration in oxidation-reduction performance of a promoter or catalyst carrier amounts to about 1,000.degree. C. as a result of efforts toward improvements in response to more and more tightened regulations.
With such conventional cerium oxide-zirconium oxide mixed oxides, however, the thermal stability of specific surface area is such that the specific surface area after heat treatment at 1,000.degree. C. is about 20 m.sup.2 /g, and the oxidation-reduction performance is such that even those mixed oxides which have not yet been subjected to heat treatment have an oxygen storage capacity of about 0.1 mmol O.sub.2 /g.
Under certain conditions of driving of automobiles (e.g. 10-mode test(emission test mode)in Japan), the majority of the total emission of unoxidized hydrocarbons is emitted at temperatures lower than the temperature at which the catalyst begins to function. This temperature at which the catalyst begins to function is called T.sub.50 (temperature at which 50% of the maximum exhaust gas purification is attained). An attempt has been made to shorten the time required to heat the catalyst by disposing the catalyst in a position close to the engine. According to this measure, the durability temperature required of the catalyst exceeds 1,000.degree. C. and, in addition, the catalyst is exposed to repeated heating cycles comprising rapid heating and cooling to room temperature. Therefore, it is necessary to develop a novel catalyst material capable of satisfying such requirements.
On the other hand, a generally known method of producing cerium oxide and zirconium oxide in the form of a homogeneously united form comprises adding a base to a cerium ion- and zirconium ion-containing aqueous solution and recovering the resulting mixed salt precipitate (Japanese Unexamined Patent Publication No.278444/1997).
However, since the mixed salt precipitate formed by the above method is a gel-like bulky hydroxide precipitate with a high water content, an extra step of filtration or solid-liquid separation is required for removing impurities. Therefore, the rate of treatment per run is necessarily reduced and, furthermore, an immense quantity of heat energy is required for thermal conversion to oxides. Thus, the conventional method of production cannot be said to be adequate for industrial scale production.